The day I meet Sonja Lyubomirsky, she keeps getting calls from her Toyota Prius dealer. When she finally picks up, she is excited by the news: she can buy the car she wants in two days. Lyubomirsky wonders if her enthusiasm might come across as materialism, but I understand that she is buying an experience as much as a possession. The hybrid will be gentler on the environment, and a California state law letting some hybrids use the carpool lane promises a faster commute between her coastal Santa Monica home and her job at the University of California, Riverside, some 70 miles inland.

Two weeks later, in late January, the 40-year-old Lyubomirsky, who smiles often and seems to approach life with zest and good humor, reports that she is "totally loving the Prius." But will the feeling wear off soon after the new-car smell, or will it last, making a naturally happy person even more so?

An experimental psychologist investigating the possibility of lasting happiness, Lyubomirsky understands far better than most of us the folly of pinning our hopes on a new car--or on any good fortune that comes our way. We tend to adapt, quickly returning to our usual level of happiness. The classic example of such "hedonic adaptation" comes from a 1970s study of lottery winners, who a year after their windfall ended up no happier than nonwinners. Hedonic adaptation helps to explain why even changes in major life circumstances--such as income, marriage, physical health and where we live--do so little to boost our overall happiness. Not only that, but studies of twins and adoptees have shown that about 50 percent of each person's happiness is determined from birth. This "genetic set point" alone makes the happiness glass look half empty, because any upward swing in happiness seems doomed to fall back to near your baseline.

"There's been a tension in the field," explains Lyubo?mirsky's main collaborator, psychologist Kennon M. Sheldon of the University of Missouri-Columbia. "Some people were assuming you can affect happiness if, for example, you picked the right goals, but there was all this literature that suggested it was impossible, that what goes up must come down."

Lyubomirsky, Sheldon and another psychologist, David A. Schkade of the University of California, San Diego, put the existing findings together into a simple pie chart showing what determines happiness. Half the pie is the genetic set point. The smallest slice is circumstances, which explain only about 10 percent of people's differences in happiness. So what is the remaining 40 percent? "Because nobody had put it together before, that's unexplained," Lyubomirsky says. But she believes that when you take away genes and circumstances, what is left besides error must be "intentional activity," mental and behavioral strategies to counteract adaptation's downward pull.

Lyubomirsky has been studying these activities in hopes of finding out whether and how people can stay above their set point. In theory, that is possible in much the same way regular diet and exercise can keep athletes' weight below their genetic set points. But before Lyubomirsky began, there was "a huge vacuum of research on how to increase happiness," she says. The lottery study in particular "made people shy away from interventions," explains eminent University of Pennsylvania psychologist Martin E. P. Seligman, the father of positive psychology and a mentor to Lyubomirsky. When science had scrutinized happiness at all, it was mainly through correlational studies, which cannot tell what came first--the happiness or what it is linked to--let alone determine the cause and effect. Finding out that individuals with strong social ties are more satisfied with their lives than loners, for example, begs the question of whether friends make us happier or whether happy people are simply likelier to seek and attract friends.